The European patent EP 0338 806 (Holman and Marshall) teaches a syringe comprising a body, a dose-setting device in the form of a rotary cap or ring mounted on the body and capable of being moved to a selected set position where a latch is arranged to retain the setting device in that set position, the movement of the setting device being accompanied by straining of a spring, which, when the latch is released, provides the force for expelling the set dose, characterized by means arranged to release the latch, which causes the return of the setting device to an original position to drive a plunger through a one-way clutch to expel the set dose; and by a quick pitch screw thread arrangement capable of transforming rotation of the setting device into linear movement of the plunger.
Also Polish patent application P 341 395 teaches a syringe for distribution of set doses of a medicine from a cartridge containing the amount of the medicine sufficient to prepare several treatment doses, comprising a housing, a piston rod having noncircular cross-section and an external screw thread, a piston rod drive arrangement comprising two elements, i.e. piston rod leaders and a nut with an internal screw thread corresponding to the piston rod external screw thread, as well as a dose-setting mechanism comprising non-self-blocking screw thread connection, along which an injection push-button is unscrewed from the nearer housing end, causing rotation of the dose-setting element. This syringe is characterized in that between the nut and the piston rod leaders there is unidirectional coupling enabling the rotation of both these parts in one direction but not in the opposite direction, wherein the allowed rotation is the only one, by means of which the piston rod is moved in the circumferential direction in the syringe. The coupling is designed in such way that the initial resistance, sufficient to resist the torque exerted on the coupling by setting a dose, has to be overcome to allow rotation. The marks indicating the size of the selected dose could be placed under the housing.
The main disadvantage of the known devices is lack of the provision of application of the precisely controlled and set dose of a medicine.
The above discussed syringe for manual application of a medicine requires from a patient the manual adjustment of the pressure force on the injection push-button, which defines the speed of the injection. This causes additional stress and discomfort at application. An additional disadvantage is a complex arrangement to transform the linear displacement of the injection pushbutton into the linear displacement of the piston rod by means of the rotary movement of the nut. Other disadvantages of a syringe for an application of a medicine using a tensioned spring comprise the failure prone construction of the trigger device, which does not ensure reliable nut blocking, and the lack of an external guiding means for the tensioned spring. This negatively affects maintaining the control and the precision of the application of the set dose of a medicine.
Besides, known devices do not ensure adequate protection against damaging the clutch or the driving arrangement due to excessive rotation of the dose-setting element. Also, forces of friction opposing the action of the plunger are too large. The arrangements for dose indication in known devices are too complex and the scale usually placed on the external surface of the housing can be easily effaced.